1. Field of the Invention
The present invention relates to a surface acoustic wave element, an electronic circuit, a frequency filter, a frequency oscillator and an electronic apparatus, and more specifically, to a surface acoustic wave element which uses a piezoelectric thin film on a sapphire single crystal substrate, and an electronic circuit, a frequency filter and a frequency oscillator provided with said surface acoustic wave element, and an electronic apparatus equipped therewith.
2. Description of Related Art
Accompanying the remarkable progress of the telecommunications field focusing primarily on cellular telephones and other mobile communications, the demand for surface acoustic wave elements is increasing rapidly. In addition, accompanying the development of an information society in recent years, the demand for not only mobile communication devices, but also for tuners capable of receiving broadcast satellite (BS) and commercial satellite (CS) broadcasts is also growing, and the proliferation of computer networks has accelerated the pace of HUB and other electrical equipment.
During the development of surface acoustic wave elements, it is important to make improvements in the three areas of compact size, high efficiency and high frequency, and in order to accomplish this, it is necessary to realize a larger electromechanical coupling coefficient (to be abbreviated as k2), a central frequency temperature coefficient that is closer to zero (to be abbreviated as TCF) and a larger surface acoustic wave propagation velocity (to be abbreviated as Vs).
In the past, a structure in which an inter-digital transducer (to be abbreviated as IDT) was formed on a piezoelectric single crystal was primarily used as a surface acoustic wave element. Typical examples of piezoelectric single crystals include quartz crystal, lithium niobate (to be abbreviated as LiNbO3) and lithium tantalate (to be abbreviated as LiTaO3). For example, in the case of RF filters that require a broader band and reduced loss of the pass band, LiNbO3 having a large k2 is used. On the other hand, in the case of IF filters requiring stable temperature characteristics even in a narrow band, quartz crystal having a small TCF is used. Moreover, LiTaO3, in which k2 and TCF are between those of LiNbO3 and quartz crystal, is used in intermediate applications.
However, since elements that satisfy all requirements of a high k2, zero TCF and high Vs cannot be realized with piezoelectric single crystal alone, a surface acoustic wave element has been proposed that uses a laminated structure including a substrate and piezoelectric thin film. This element combines a substrate and thin film material, and allows the obtaining of characteristics superior to single crystal by controlling the orientation direction of the thin film and film thickness. For example, by forming a piezoelectric thin film having a large k2 on a substrate having a TCF near zero, it is possible to realize a surface acoustic wave element for which the overall characteristics include a high k2 and zero TCF.
Here, in the case of considering the use of a surface acoustic wave element as an oscillator in particular, although specific examples of the oscillators include high order increase types, phase synchronization types, and direct oscillation types, the direct oscillation types are desirable in order to reduce the size of the element and improve jitter. Therefore, it is important to increase the oscillation frequency. In the case of increasing the oscillation frequency, an element structure that realizes a higher frequency by laminating a piezoelectric layer with a hard layer having a high Vs is promising even in consideration of limits on the design rule of the electrode pitch of the IDT. In Japanese Unexamined Patent Application, First Publication No. Hei 6-164294, a diamond film is formed as a hard layer on an Si substrate, onto which a ZnO piezoelectric layer and SiO2 protective layer are additionally formed, by which an extremely important structure satisfying all of the requirements of high k2, zero TCF and high Vs can be obtained.
However, the surface acoustic wave elements having the hard layer and the piezoelectric layer of the prior art had the following problems.
In the case of using the diamond thin film hard layer described in Japanese Unexamined Patent Application, First Publication No. Hei 6-164294, although high Vs can be obtained, surface flatness cannot be obtained. Under the present circumstances, in addition to the polishing process being difficult and requiring considerable time due to the hardness of the diamond thin film, since the diamond film is polycrystalline, surface flatness is considerably inferior to that of ordinary piezoelectric single crystal substrates. Thus, the crystallinity of the piezoelectric thin film deposited on the diamond thin film also decreases, thereby preventing the obtaining of an epitaxial film. The characteristics of the piezoelectric thin film as surface acoustic wave elements are greatly dependent on its crystallinity, and in the case of using the polycrystalline diamond thin film hard layer, problems occur including a decrease in k2 in particular.
Thus, at the time of practical application, it is important to attempt to increase Vs by a single crystal hard layer other than diamond. Sapphire is one example of a hard layer and a substrate material other than diamond which is a single crystal, is easily obtainable, and has a large Vs. Although the Vs of sapphire is inferior to that of diamond, which has a Vs of about 10000 m/s, its Vs is still large, and its single crystal substrate is inexpensive and is used universally. Since various types of piezoelectric thin films also demonstrate epitaxial growth on such substrates, it is an extremely important material. Here, since the propagation velocity of a surface acoustic wave (Vs) is Vs xe2x88x9d (E/xcfx81)xc2xd when Young""s modulus is represented with E and density is represented with xcfx81, if it were possible to increase the Young""s modulus and decrease the density of sapphire through the use of additives, it would be possible to increase its propagation velocity to a level comparable to that of diamond.
The present invention is accomplished in view of the above circumstances, and aims at providing a surface acoustic wave element which uses a sapphire hard layer having a large Vs and which is able to accommodate higher frequencies of the band used, and providing an electronic circuit, a frequency filter, and a frequency oscillator, each of which is provided with said surface acoustic wave element, and an electronic apparatus equipped therewith.
In order to solve the above problems, the present invention provides a surface acoustic wave element including a sapphire single crystal substrate, a hard layer formed on said sapphire single crystal substrate and having a composition containing (Al1-xM1x)2O3 (0xe2x89xa6xxe2x89xa60.5)) in which at least one element M1 (M1=B, Ga, In, Ti, V, Cr, Mn, Fe, Co) is added to sapphire, and a piezoelectric layer formed on said hard layer.
According to the above composition, Vs of the surface acoustic wave is increased by either increasing Young""s modulus or decreasing density of the hard layer, thereby enabling the surface acoustic wave element to be used in high frequency regions.
In addition, the above hard layer can have a corundum crystal structure.
According to this composition, as a result of conforming the lattice to a single crystal substrate in which the hard layer has the same crystal structure, the crystallinity of the thin film is improved and k2 is improved, thereby making it possible to conserve the electrical power of the surface acoustic wave element.
In addition, the above piezoelectric layer can have a composition containing (Al1-xBx)N (0xe2x89xa6xxe2x89xa60.5) in which boron nitride is added to aluminum nitride, and also can have a wurtzite crystal structure.
According to this composition, together with increasing the Vs of the surface acoustic wave by either increasing the Young""s modulus or decreasing the density of the aluminum nitride, by conforming the lattice to the hard layer by reducing the lattice constant of the aluminum nitride, the crystallinity of the thin film is improved and k2 is improved, thereby making it possible to increase the frequency and conserve electrical power of the surface acoustic wave element.
In addition, the above piezoelectric layer can have a composition containing (Zn1-xM2x)O (1xe2x89xa6x greater than 0.5) in which at least one element M2 (M2=Li, Mg, Fe, Co., Ni, Cu) is added to zinc oxide, and can also have a wurtzite crystal structure.
According to this composition, together with increasing the Vs of the surface acoustic wave by either increasing the Young""s modulus or decreasing the density of the zinc oxide, by conforming the lattice to the hard layer by reducing the lattice constant of the zinc oxide, the crystallinity of the thin film is improved and k2 is improved, thereby making it possible to increase the frequency and conserve electrical power of the surface acoustic wave element.
In addition, the present invention provides a surface acoustic wave element including a single crystal substrate having a composition containing (Al1-xM1x)2O3 (0xe2x89xa6xxe2x89xa60.5) in which element M1 (M1=B, Ga, In, Ti, V, Cr, Mn, Fe, Co) is added to sapphire and having a corundum crystal structure, and a piezoelectric layer formed on said single crystal substrate.
According to this composition, the surface acoustic wave element can be used at a higher frequency region by increasing the Vs of the surface acoustic wave by either increasing the Young""s modulus or decreasing the density of the sapphire single crystal substrate.
In addition, the above piezoelectric layer can have a composition containing (Al1-xBx)N (0xe2x89xa6xxe2x89xa60.5) in which boron nitride is added to aluminum nitride, and can also have a wurtzite crystal structure.
According to this composition, together with increasing the Vs of the surface acoustic wave by either increasing the Young""s modulus or decreasing the density of the aluminum nitride, by conforming the lattice to the hard layer by reducing the lattice constant of the aluminum nitride, the crystallinity of the thin film is improved and k2 is improved, thereby making it possible to increase the frequency and conserve electrical power of the surface acoustic wave element.
In addition, the above piezoelectric layer can have a composition containing (Zn1-xM2x)O (0xe2x89xa6xxe2x89xa60.5) in which at least one element M2 (M2=Li, Mg, Fe, Co., Ni, Cu) is added to zinc oxide, and can also have a wurtzite crystal structure.
According to this composition, together with increasing the Vs of the surface acoustic wave by either increasing the Young""s modulus or decreasing the density of the zinc oxide, by conforming the lattice to the hard layer by reducing the lattice constant of the zinc oxide, the crystallinity of the thin film is improved and k2 is improved, thereby making it possible to increase the frequency and conserve electrical power of the surface acoustic wave element.
In addition, in order to solve the above problems, the present invention provides a frequency filter including a first electrode on the above piezoelectric layer equipped with any one of the surface acoustic wave elements described above, and a second electrode which is formed on said piezoelectric layer and which converts to an electrical signal by resonating to a specific frequency or a frequency of a specific band of a surface acoustic wave generated in said piezoelectric layer by an electrical signal applied to said first electrode.
According to this composition, since Vs of the surface acoustic wave is high, the electrical signal of the specific frequency or the frequency of the specific band can be filtered by applying an electrical signal of a higher frequency to the first electrode.
In addition, in order to solve the above problems, the present invention provides a frequency oscillator including an electrical signal application electrode which is formed on the above piezoelectric layer equipped with any one of the surface acoustic wave elements described above and which generates a surface acoustic wave in said piezoelectric layer according to an applied electrical signal, and a resonance electrode formed on said piezoelectric layer which resonates a specific frequency component or frequency component of a specific band of the surface acoustic wave generated by said electrical signal application electrode.
According to this composition, since Vs of the surface acoustic wave is high, in addition to being able to generate a surface acoustic wave by applying an electrical signal of a higher frequency to the electrical signal application electrode, a surface acoustic wave of a higher frequency can be resonated. As a result, signals of higher frequencies can be oscillated.
In addition, in order to solve the above problems, the present invention provides an electronic circuit including the above frequency oscillator, and an electrical signal supply element which applies an electrical signal to the above electrical signal application electrode provided in said frequency oscillator, wherein a specific frequency component is selected from the frequency components of said electrical signal or the frequency components of said electrical signal are converted to a specific frequency component, or a prescribed modulation is imparted to said electrical signal followed by a prescribed demodulation or prescribed wave detection.
According to this composition, since a specific frequency component is selected from signals of a higher frequency, or signals of a higher frequency are converted to a specific frequency component, or a prescribed modulation is imparted to said electrical signal followed by a prescribed demodulation or prescribed wave detection, an electronic circuit can be provided that electrically performs various operations on an electrical signal of a higher frequency.
In addition, in order to solve the above problems, the present invention provides an electronic apparatus that contains at least one of the above frequency filter, the above frequency oscillator, and the above electronic circuit.
According to this composition, since at least one of the frequency filter, the frequency oscillator, and the electronic circuit that operate at a higher frequency is contained, mobile communication devices, satellite broadcast tuners and various other electronic apparatuses can be provided in which higher frequencies are scheduled to be used in the future.